A Geodynamic Study For The Deep Structure And Plateau Growth In Northeastern Tibet

Cenozoic collision of the Indian plate and the Asian plate caused formation of the magnificent plateau of Tibet. Previous studies suggest that continental collision and the subsequent subduction lead to lithospheric deformation and growth of the Tibetan plateau. The effects of this interaction between the Indian and Asian continents on northeastern Tibet (NE Tibet), the tectonic transition zone between the Tibetan plateau and the North China craton (NCC) for example, remain uncertain due to inadequate geophysical data coverage in northeastern Tibet. Thus more dense data coverage is necessary to be added to the dataset of observation. Here in this thesis, based on new dataset collected from a dense linear array of 38 broadband seismograph stations, we applied seismic receiver functions (Sp and Ps converted waves) to imaging the lithospheric structure and shear wave splitting (XKS waves) to inspecting the anisotropy in the lithosphere and upper mantle beneath NE Tibet. The seismic array traverses NE Tibet to the westernmost NCC in an SSW-NNE direction. Under joint constraints from both the lithospheric structure imaging and the regional anisotropic regime, combined with previous studies and through a thorough analysis/comparison/integration, we finally constructed a comprehensive lithospheric model of NE Tibet. The model tells that the NCC lithospheric mantle has been persistently underthrust beneath the Qilian orogen in response to on-going convergence/compression between the interior Tibetan plateau and the NCC. This process forms the syntectonic crustal thrust. The regional anisotropic regime can be well accommodated in our interpretation. The lithospheric model summarized here can be well accommodated in a scenario of northeastward migration of stepwise/multiple Aisan mantle lithosphere underthrusting beneath the Tibetan plateau. The multiple Aisan lithospheric blocks underthrust the plateau stepwise in small scale.Our results are based on several significant findings:1. The lithosphere-asthenosphere boundary (LAB) is clearly defined and appears as a south-dipping interface that runs continuously from the Alxa interior to the Qilian orogen on the S-wave receiver function images. Shear wave splitting measurements show significant lateral variations of seismic anisotropy across NE Tibet.2. Combined with previous tomography studies and our receiver function images, the SKS travel-time analysis along the array supports the inference that a cold/rigid Asian lithosphere resides beneath the Qilian and Alxa blocks while a hot/soft Tibetan lithosphere resides beneath the Songpan-Ganzi (SPGZ) and Kunlun-West Qinling (KL-WQL) blocks. The observed variations of anisotropy along the array indicate the important role the major faults have been playing in the process of lithospheric deformation in NE Tibet. The West Qinling fault (WQLF) is reckoned as the boundary between the Tibetan lithosphere and the Asian lithosphere in the study area. The Kunlun fault to West Qinling fault (KLF-to-WQLF) zone may constitute a boundary accommodating the eastward extrusion of the Tibetan lithosphere, with the rigid Asian lithosphere in the north barrier to the northeastward tectonic flow of central-eastern Tibet.3. A significant character of two-layer anisotropy was identified in the Qilian orogen, which was inferred to be associated with the LVL acting as a thrust decollement in the mid-to-lower crust. A thorough analysis involving crustal anisotropy and the regional XKS splitting results in NE Tibet, in association with the crustal LVL feature, indicates that decoupling deformation may dominate the lithosphere beneath the Qilian orogen while coherent deformation may dominate the lithosphere beneath the WQL and SPGZ blocks. Anisotropy beneath the Qilian orogen seems consistent with recent deformation under the boundary forces related to possible lithosphere underthrusting from its northern margin.4. P-wave receiver function imaging reveals several significant intracrustal features, including a north-dipping intracrustal converter (NC) beneath the West Qinling orogen, Moho offset/overlap beneath major fault zones such as the west Qinling fault and the Haiyuan fault, and a low velocity layer (LVL) in the middle-lower crust beneath the Qilian orogen.Overall, our results provide a new section from crust to mantle and a more thorough view of lithospheric structure and deformation on NE Tibet to help enhance our understanding of the Cenozoic post-collisional evolution of the tectonic transition zone between NE Tibet and the NCC and clarify the mechanism of plateau growth in this boundary area.